Front-end amplifiers for optical receivers are traditionally realized in the form of transimpedance amplifiers which provide a good compromise between high sensitivity and wide dynamic range. However, to achieve the best possible performance from new transistor technologies and low capacitance photodiodes, a standard transimpedance amplifier configuration is often not adequate. It would be desirable to have an amplifier that allows very high overload combined with sensitivity that is limited only by the intrinsic noise of the input transistor, thus reaching the theoretical limit for PIN/FET receiver performance.
It is well known that to achieve minimum sensitivity, it is necessary to minimize the input capacitance of an optical receiver, and to maximize the value of feedback resistance in the transimpedance amplifier. The difficulty in accomplishing this in practice comes from the large values of feedback resistors needed. Large feedback resistors cause at least three separate problems:
Overload capability is inversely proportional to the feedback resistor value, hence large resistors reduce dynamic range; and PA1 Transimpedance amplifier bandwidth is also inversely proportional to feedback resistor value, hence large resistors reduce bandwidth. Bandwidth can be improved by increasing the open loop gain of the transimpedance amplifier, but the increase in gain is limited by increasing amplifier phase shift which gives rise to frequency response peaking and associated sensitivity degradation; and PA1 Shunt capacitance in parallel with the feedback resistor can eventually limit the transimpedance amplifier bandwidth as the feedback resistor value grows.
In the current art, the problem of dynamic range is sometimes overcome by the use of FETs acting as voltage-variable resistors to shunt photocurrent away from the transimpedance amplifier input when high optical signal levels are received. A FET shunt can be configured either a) in parallel with the feedback resistor, or b) as a shunt from the transimpedance amplifier input to ground. With an extremely large feedback resistor and very small input capacitance, the utility of the shunt FET approach is limited. A FET in parallel with the feedback resistor adds shunt capacitance and limits the bandwidth. Bandwidth could potentially be restored by equalization, but nonlinearity of the FET would limit the equalizer's effectiveness. As the shunt is turned on with increasing signal levels, it would be necessary to reduce the open-loop gain of the transimpedance amplifier to prevent peaking. Because of the high open-loop gain needed with a large feedback resistor, the circuit would already be at or beyond the phase shift limit where peaking becomes a problem. The difficulties of matching the shunt FET resistance and open loop gain over temperature, optical power range, and process variation would be extremely formidable.
Alternatively, a shunt FET to ground might be used. However, the ON resistance of this FET would have to be much lower than that of an FET used as a feedback resistor shunt, necessitating a much larger device size. A large FET would add considerable parasitic input capacitance, hence reducing the receiver sensitivity. Calculations have shown an expected degradation of 1.5 dB, which would significantly reduce the range of application of the receiver.